Apparatus for measuring the concentration of solutions



Jan. 19, 1937. F. H. MACKENZIE 2,068,499

APPARATUS FOR MEASURING THE CONCENTRATION OF SOLUTIONS Filed March 5, 1955 2 Sheets-Sheet l Jan. 19, 1937. F. H. MACKENZIE 2,068,499

APPARATUS FOR MEASURING THE CONCENTRATION OF SOLUTIONS Filed March 5, 1955 a Sheets-Sheet 2 ATTORNEYS Patented Jan. 19, 1937 UNITED STATE ammo mm'rcs l-on masvamc rm:

coucan'ras'rrou orsowrrons Franklin n. Mackenzie, Bywood, PL, um to American Chemical raintcombany, Ambler,

Pa., a corporation of Delaware Application March 5. issa'semi No. 9.51s 2 Claims- (Cl. 175-183) This invention relates to apparatus for measuring the concentration'or solutions. 4

One of the primary objects of the invention v 10' be mentioned the arrangement of the equipment for operation on common and readily available sources of current such, for instance, as the ordinary 110 v., 60 cycle alternating current supply line; the arrangement of the equipment to 15 provide direct concentration readings; the provision of means for compensating for temperature changes of the solution measured; the provision ,of' means for compensating for variations in voltage of the supply line; and the, provision 20 of means for compensating for diflerences in the conductivity of the solvents used in preparing solutions. v c

How the foregoing objects and advantages are secured, together with others which will occur to those skilled in the art will be apparent from a consideration of the following description taken in connection with the accompanying drawings illustrating several embodiments of the invention, and in which-- 30 Figure 1 is a diagram ,of one embodiment of the invention which is especially suitable for the measurement'of solution concentration in ranges in which the electrical conductivity is relatively high and in which changes of conductivity resulting from temperature variation are relatively small as compared with conductivity changes resulting from variation in concentration:

- Figure 2 is a diagram of an embodiment of 40 the invention especially suitable for the measurement of concentration of solutions in ranges where the conductivity changes due to temperature variations are relatively large; i

Figure 3 is a view similar toFigure 1 but 4,5 illustrating a modification; I

- Figure 4 is a view similar to Figure-2 but illustrating a still further modification, and Figures 5' and 6 are-views illustrating the type of calibration adopted for certain instruments 50 employed." a

' In the following description of -the'- several diagramsit is to be understood that the voltage, resistance and othervaluesgiven are 'onlyby way of example, since' ithe" desiredmesult may 55 still be secured even where the departure from the suggested values is relatively great. Furthermore, modifications of this type will necessarily be made in many instances to accommodate the equipment to measurement of concentration of widely diilering solutions.

In Figure 1 a powerline, such as the common 110 v., 60cycle supply, is indicated by the numerals 5 and 6. While this potential may be employed directly, I prefer to use a step-down transformer inorder to reduce the voltage in the 10 equipment to a point, for example, 12 v., which is safer for ordinary handling; Such a transformer appears at the left of Figure 1, and is shown as having its primary 1 connected across the power line and its secondary 8 coupled with 15 the equipment now to be described.

Any suitable make-and-break switch 9 may be employed in the instrument circuit lil which, of course, receives its current from thesecondary 8. The circuit In may be connected with a pair of electrodes il either directly or by switch l2, the purpose of which latter is to provide for the alternative connection of any one of a number of other pairs of electrodes such, for instance, as that shown at H. In this way the equipment may be used in the measurement of concentration of solutions in a multiplicity of different receptacles, each having a pair of electrodes ll. As to the electrodes themselves, these are preferably-made of some material not subject to attack by the ingredients of the particular solution to be measured. The electrodes; of course, will also be insulated from each other and preferably mounted in fixed relative positions, 1 e., so mounted as to maintain the spacing constant.

If desired, a large tank may be equipped with more than one pair of electrodes, these being located at different points, for instance, one toward the bottom ,and one toward the top, so that the concentration in difierentzones may be measured.

In the arrangement of Figure 1 a voltmeter I3 is coupled in parallel with electrodes H, and during operation this meter will respond to fluctuations of voltage across circuit in, which fluctuations, of course, correspond with the conductivity and thus the concentration; of thesolution.

In the arrangementof Figured the meter. may be a common ApG. voltmeter of about 10 v. range, and I prefer -to,calibrate this meter. in units; of concentration; The calibration of con centration units as indicated in Figure 5, will be inverted withreference to the voltage calibration since, in; general, the voltage will decrease "as a the .concentration;(andt conductivityii creases, and vice verse. Note further that 1 prefer to choose a meter having a voltage range of a spread considerably greater than necessary insofar as the calibration of concentration is concerned, this for the reason that the common types of A. C. voltmeters are quite sensitive and accurate in about the uppe half of the voltage range, although the accuracy'dec'reases appreciably toward the lower end of the range. Thus, in the preferred meter arrangement the minimum concentration indication will be placed at the high-voltage end of the scale. and the max-'- imum concentration indication will be placed in the neighborhood of the middle. of the scale.

The minimum concentration, of course, will correspond with the conductivity of the solvent used in preparing the solutions, and for purposes of identification I prefer to mark the "zero" end of the concentration graduations as water.

With this equipment, the flow of current in the circuit i0 and between the electrodes ii in a solution will aiford a concentration reading on the voltmeter. There are, however, certain factors which must be compensated for in order to obtain an accurate reading. In the first place, the normal voltage of the power line may be diiierent in different localities and the voltage may vary from time to time in accordance with fluctuations in the supply line, and with this in mind I introduce a variable impedance or resistance inthe circuit it. Inthe arrangement of Figure 1 this device takes the form of a variable rheostat it having, for example, 20 ohms maximum resistance. This rheostat serves to vary the voltage appliedto the electrodes and may be employed not only to correct for fluctuations in the voltage of the supply line but also to compensate for differences in the conductivity of the solvent employed in-making up solutions. Compensation for these factors may be accomplished by immersion of electrodes ll into a receptacle containing pure solvent. In an industrial-plant this solvent will quite usually be derived from supply, an adjustment oi'jthe device It would benecessary only very rarely.

Another important factor which must be com-' pensated for in order to secure accurate readings is the temperature of the solution being measured. In accordance with this invention, this compensation is taken care of by the use of. an additional resistance or equivalent device It which.-as'sh wn in Figure 1, may slutably'take' the form of a 20 ohm rotary rheostat. To fa cilitate use of the equipment in taking measure ments, I prefer to calibrate this-rheostat it in units of temperature so that the operator need only take a temperature reading from the solution being measured and then adjust rheostat II to the corresponding value.

The initial calibration of the device I! may be accomplished in the following manner:

Assume that the equipment of Figure .1 is to be used for the measurement of concentration of common salt in water. For this purpose, the adv justment of resistance It referred to above is first carried out while the electrodes ii are: immersed in the water to be employed as solvent.

rect reading of concentration Preferably this sample of the water i mist to a known temperature at about the middle of therangeoverwhichthesolutionistobeallowed tovary. Whilethisisbeingdonerheostat it is set at its minimum value.

The rheostat II is now moved to a point near themiddleofitsrangeandtheelectrodes II are plscedin' a salt solution of known concentration at a known temperature, preferably at about the middle of the range to be encountered. The needle of the meter it is observed and a suitable 1 marking is applied to its scale. This operation may be repeated several times with sample salt solutions at the same known temperature but of diflerent concentration, and in each instance suitable marking is applied to the scale of meter and the temperature thereof brought to a plu-; rality'of different points within the range to be covered. At each temperature the rheostat II is adjusted to bring the meter needle to the proper concentration marking and the position of the rheostat control is marked on the scale of the rheostat. A typical calibration of the rheostat I5 is shown in Figure 6.

It may here be noted that since, in general, the resistance of electrolytes falls with rising temperature, the temperature calibrations on the scale of rheostat ll will be arranged to bring the highest temperature reading adjacent the highest resistance value of the rheostat.

Modifications of the'procedure in calibrating might be made. For example, the order of calibration might be inverted and the rheostat ll calibrated first. with further reference to the matter of calibration it is to be understood that various expressions such as "calibration and calibrated in units of concentration" (or temperature) are used in a broad sense. Obviously any arbitrarily chosen indicia or markings may be adopted. The calibrations appearing in Figurea 5 and 6 are illustrative only.

In any event, the calibration of the scale of rheostat ii in degrees of temperature and of the scale of meter ii in units of concentration may readily be accomplished by employing sample solutions of known concentration and temperature and following through a series of manipulations as suggested above. It should further be understood that the calibration must necessarily be different with diflerent electrolytes, although if desired, multiple calibrations for different electrolytes may be applied to the scales of the meter I3 and rheostat I5. Once the initial calibration is completed, the apparatus may very readily be employed to secure direct readings of concentration, the manipulations necessary being extremely simple and therefore virtually foolproof. Assuming, by way of example. that the equipment of Figure -1 is calibrated for the measurement of solutions of common salt, the operator need only immerse a pair of electrodes ii in any particular solution to be measured. determine the temperature of this solution and then adjust rheostat II to that temperature. A diwlll now be given by the meter i3.

As indicated above, the layout of Figure l is especially suitable for the measurement of solutions having relatively high conductivity and in which variation in conductivity as a result of 1| accents.

temperature change is relatively slight as compared with variation in 9 diictivityas a result prefer a somewhat modified arrangement ofthe readily be seen, this is somewhat diiferent than compensatin equipment, as shown, for instance, in Figure 2. Here" again a ste -down transformer coupled with the, supply line H is also employed. In this instance the primary and secondary windings it and, I1 maydesirably be'proportioned to deliver a slightly higher voltage to the measuring apparatus, for example, in the neighborhood of 20 v. The instrument circuit .IC is also 'pr'ovided'with a switch I as-well as a resistance or equivalent device llserving a purpose somewhat similar to that of the resistance ll of Flip.

1. In this arrangementthe meter ll, which may also be of about 10 v. range, has a series connection with the electrodes it". As will thelayout of Figure 1. and in this instance I prefer to employ 7 range, for example, 100 china-as the'means for for temperature fluctuations. It willfbeobserved, however, that in both of Figures 1 and 2 the rheostat used for compensation of temperaturefluctuations is in series'both with the-meter ll and'with the electrodes 1 l. r

In the showing of Figure 2, while only one pair ofelectrodes l'i are illustrated, any suitable number may be provided and these, of cause. may alternatively be. coupled into the circuit It by means of a switch 20. I prefer to provide for the connection of a resistance 2| into circuit I. in place of a pair of electrodes Ii. and this may also be accomplished by the switch 20 as shown. Resistance 2| may have a value in the neighborhood .of 60 ohms and is employed only for calibration purposes in accordance with the following. In this case, assume that the. apparatus is to be used -in measuring the concentration of sodium arbonate in water.

with the resistance 21 placed in the circuit is and with the switch 2 closed, the temperature compensating rheostat I9 is preferably .set in the neighborhood of its mid-point and the re'-,

sistance I4 is then adjusted until the meter 13 registers maximum voltage.

In the initial setting and calibration of the equipment of Figure 2, the meter I3 is callbrated in units of concentration over a range corresponding with the range which it is desired to measure, and because of the connections here employed the minimum point on the concentration scale will coincide with the minimum voltage reading, and the maximum concentration point will coincide with the maximum voltage reading.v Furthermore, the rheostat I9 is callbrated over the range of temperatures which must be compensated for, for example, from 60 F. to 140 F. Because of the diiIerence in the connections of vFig. 2 as compared with Fig. 1, the highest temperature reading on the scale of rheostat I! will be placed adjacent the minimum resistance value. v Thus it will be seen that the mid-point -(to which the rheostat i9 is set when commencing calibration) will correspond to about 100? F.

Subsequent to the initial adjustment of rheostat H, as mentioned above, the resistance 21 is taken out of the circuit and a pair of electrodes Ii are immersed in a sample of the water to be used as a solvent. The temperature of a resistance it having a largethissampleis adjustment the needle of the meter 11 is set to the zero mark (minimum concentration), as by means of the meter needle-reset screw, provided on such instruments.

, noted and the rheostat ll adiusted to the corresponding value. After this The foregoing procedure provides calibration coupled with the su ply line 8-6 by means ofan autotransformer 22 having a multiplicity of taps providing for voltage and impedance variation in the circuit 2|. Rheostat II is again included in this layout for the purpose discussed above in connection with Figure 1.

The calibration of the arrangement of Figure 3 will be essentially thesame'as'thatfor Figure 1, although the temperature values will be applied to the several taps of the transformer 22. As to Figure 4,- the general operation and calibration are similar to that for'Flgure 2. Here the rheostat 14' will again be employed for the initial calibration step when the resistance 21 is placed in the circuit. A variable impedance 23 is substitutedin this system for the resistance 19, although its function and general operation are quite similar.

Figure 4 'also illustrates apparatus for direct connection toa low voltage A. C. supply-line '6'.' The voltage in this line may be in the neighborhood .of about 15 in As a further modification, Figure 4 illustrates the adaptation of the of- Figure 3, especially as the application of a key 24, in place of switch 9 employed in Figures 1 to 3.

From the foregoing it will be seen that my improvement is capable of numerous variations without, however, departing from the general type of operation herein contemplated and while retaining at least most of the advantages, especially the obtaining of direct concentration readings which are properly corrected for temperature variations, line voltage fluctuation and differences in the purity or conductivity of solvents employed.

I claim: I

1. Apparatus of the type described including, in combination with a source of electrical-current, a pair of spaced electrodes adapted to be immersed in solutions of varying concentration, a circuit connecting said electrodes with said source, a voltmeter connected in parallel with said electrodes, a variable electrical device in said circuit in series with the electrodes and with the meter for compensating for iiuctua- 1 solutions being measured.

2. Apparatus of the type described including,

combination with a source of electrical current,

a pair of spaced electrodes adapted to be immersed in solutions of varying concentration, a

circuit connecting said electrodes with said source, a voltmeter connected in parallel with said electrodes the meter being calibrated in 4 2,068,489 units of concentration, a variable resistance in also in series with the electrodes and pith the said circuit in series with the electrodes and. meter for compensating for variations in elecwith the meter for compensating for fluctuatrical conductivitrot the solvent used in pretions in temperature or solutions to be measparing the solutions being measured.

I ured, the resistance being calibrated in units of temperature, and a second variable resistance FRANKLIN H. 

